It is difficult to imagine a world without automobiles. From school buses to taxis to the family sedan, automobiles have become an integral part of daily life for most individuals. Arguably the most important element of the automobile is its engine. Almost all engines are internal combustion engines that have one or more cylinders. A piston is movably positioned within the cylinder and connected to a crankshaft by a rod. The movement of the piston turns the crankshaft providing motive force to the wheels of the vehicle.
Engines typically operate using a four stroke combustion cycle, also known as the Otto cycle. The four strokes are: 1) the intake stroke, where the piston moves from an in position to an out position, drawing fuel and air into the cylinder; 2) the compression stroke, where the piston moves from the out position to the in position, compressing the fuel and air mixture in the cylinder; 3) the combustion stroke, where a spark plug emits a spark, igniting the fuel and air mixture, causing an explosion that propels the piston to the out position; and 4) the exhaust stroke, where the piston moves back to the in position, forcing the exhaust fumes from the cylinder. Valves permit the introduction of fuel and air, and another valve allows the expulsion of the exhaust fumes.
This cycle is made inefficient when a misfire occurs. A misfire is commonly thought of by those in the automotive industry as the total lack of combustion in the cylinder. Over the years, the automotive industry has identified several suspects that can cause misfires. The first primary suspect is the spark plug. The spark plug may not be providing a spark to the compressed fuel/air mix and thus, no combustion happens. The second primary suspect is a bad fuel mix. This may be the result of a clogged fuel injector, a clogged air intake, or both. Essentially, the fuel and the air are not being introduced into the cylinder in the proper ratio to achieve combustion. The final primary suspect is a lack of compression, such as when the seals in the cylinder fail and allow the fuel/air mixture to leak out as the piston attempts to compress the mixture. Those skilled in the art will recognize that there are other reasons for a lack of combustion, and that there may be other causes even within the primary suspects outlined here. For example, a bad fuel mix may be the result of an empty gas tank.
Misfires are a cause for concern for vehicle operators. When a misfire occurs, the fuel in the cylinder does not combust and is sent out the exhaust valve. This unburned fuel then passes out through the exhaust system of the vehicle. This results in lower fuel efficiency as well as additional wear and tear on catalytic converters. Thus, vehicle operators may take their vehicle to a service technician for repair if the operator suspects that the engine is misfiring. However, generally knowing what may cause a misfire does not greatly help the service technician identify which cylinder is misfiring.
The United States government has decided that misfires are a cause for concern relative to the environment. Specifically, the unburned fuel that is expelled through the exhaust system of the vehicle may contribute to air pollution. In 1996, legislation was passed that required vehicle computers to perform onboard detection of misfiring. Further, this legislation requires that the cylinder that is misfiring be pinpointed and identified by the onboard computer. Initial efforts to detect misfires focused on the crankshaft and its rotational speed. The theory behind these efforts was that a misfire would cause the crankshaft to slow down momentarily. However, it was soon discovered that vibrations, such as those caused by bumpy roads, also caused the crankshaft to slow down, which could result in a false positive. As a result, the regulations that implemented the legislation were eased to allow for less sensitive sensors. Now the sensors operate with a timing threshold and indicate a misfire if a periodic slowing of the crankshaft occurs for more than a predetermined amount of time. These sensors may still report false positives, and further, these sensors do not report combustion inefficiencies short of a total misfire. More information on this topic can be found at Carley's “Decoding Onboard Diagnostics,” submitted in the Information Disclosure Statement. Thus, there is still a need for a technique that allows compliance with the 1996 statute.
While the detection of misfires is certainly a laudable goal, it is not the only thing that the automotive industry has done to reduce pollution. Catalytic converters were introduced to burn non-combusted fuel in the exhaust path. Catalytic converters store oxygen therein and use this oxygen to burn fuel that was not burned in the engine. By burning fuel in the catalytic converter, fewer hydrocarbons are released into the atmosphere.
It has further been discovered that the optimal combustion of fuel occurs when the air to fuel ratio is 14.7:1 while the engine is idling. This ratio is sometimes called lambda. To avoid placing undue strain on the catalytic converters, a lambda sensor is placed in the exhaust system. The lambda sensor senses oxygen levels in the exhaust and communicates with the vehicle's onboard computer via a voltage signal. The voltage signal of the lambda sensor decreases as oxygen levels increase and increases as oxygen levels decrease. Because the lambda sensor detects oxygen levels, it is sometimes called an oxygen sensor. The onboard computer examines the level of oxygen and infers that the air to fuel mixture is too lean or too rich. The onboard computer can then adjust the fuel injectors and/or the air intake systems to provide the desired air to fuel ratio.
To date, no one has used the lambda sensors to detect misfires in the engine, and it has been hypothesized that it would be impossible to link the lambda sensor to a particular cylinder misfiring.